1. Field
It is related to a control circuit for a DC-DC converter.
2. Description of the Related Art
Recent electronic devices such as a personal computer use DC-DC converters as power supplies. In such a DC-DC converter, high conversion efficiency is required for a wide load range extending from a heavy load in which a large amount of power is supplied to a light load in which a small amount of power is supplied.
A battery is installed in a portable electronic device as a drive power supply. The output voltage of the battery is decreased by device usage and discharging. Thus, a direct current voltage conversion circuit (DC-DC converter) for converting the voltage of the battery to a constant voltage is arranged in the electronic device. A synchronous rectification type DC-DC converter, which is compact and has satisfactory conversion efficiency, is used for a portable electronic device. A typical synchronous rectification type DC-DC converter performs pulse width modulation (PWM) and alternately activates and inactivates a main switching transistor and a synchronous transistor. Specifically, energy is supplied from an input side to an output side of the DC-DC converter when the main switching transistor is activated, and energy accumulated in a choke coil is discharged when the main switching transistor is inactivated. The synchronous transistor is inactivated in synchronization with the timing the energy accumulated in the choke coil is discharged to the load. The DC-DC converter controls the pulse width of a pulse signal that drives the main switching transistor in accordance with the output voltage or output current to keep the output voltage substantially constant.
In the above-described DC-DC converter, high conversion efficiency is required for a wide load range extending from a heavy load in which a large amount of power is supplied to a light load in which a small amount of power is supplied. However, a relatively large power loss occurs in the DC-DC converter compared to the power consumption at the load when driving the main switching transistor under a light load. It is generally known that this significantly lowers the conversion efficiency.
To prevent the lowering of the conversion efficiency under a light load, Japanese Laid-Open Patent Publication No. 6-303766 proposes a DC-DC converter that switches from pulse width modulation (PWM) to pulse frequency modulation (PFM) when the load is light. The DC-DC converter performs PWM under normal operations, which include operations under a heavy load, and PFM when the load is light. The PFM includes true PFM, which controls the switching frequency of the drive signal provided to the main switching transistor in accordance with the output voltage of the DC-DC converter, and pseudo-PFM, which keeps the switching frequency constant and reduces switching operations in accordance with the output voltage of the DC-DC converter. In both true PFM and pseudo-PFM, the switching frequency under a light load becomes lower than that during PWM. This reduces the power loss of the DC-DC converter. Accordingly, this suppresses the lowering of the conversion efficiency when the load is light.
It is generally known that a DC-DC converter that switches between PWM and PFM holds a constant output voltage while maintaining high conversion efficiency in a wide load range. However, if the main switching transistor is inactivated under a light load, reverse current may flow from the load towards ground via the synchronous transistor. This results in the loss of the energy accumulated in the choke coil and lowers the conversion efficiency when the load is light.